Every modern-day implantable pacemaker includes a sensing circuit for detecting cardiac events (herein-after referred to as an "event detector"), whether the activity of one or both chambers of the heart is sensed. The electrical cardiac signal, as recorded inside the heart, is called an intracardiac electrogram, IEGM, (or simply, an electrogram, EGM) and is a very rapid, relatively large signal. The most rapid portion of this signal is called the intrinsic deflection, and this is what the pacemaker senses. The pacemaker can sense the intrinsic deflection portion of one or both of the atrial or ventricular electrogram (EGM) from within the heart. The atrial EGM coincides with the P-wave of the surface ECG, while the ventricular EGM coincides with the R-wave of the surface ECG. For purposes of simplicity and consistency with common usage, the terms P-wave and R-wave, as used herein, are synonymous with the intrinsic deflection portion of the atrial and ventricular electrograms, respectively.
In practice, the cardiac signal is amplified within the event detector by a sense amplifier, and the sensitivity level of the pacemaker is proportional to the gain of the sense amplifier. A cardiac event is sensed when an amplified cardiac signal from the sense amplifier exceeds a threshold level. The sensitivity level of the pacemaker and the threshold level are selected such that only the amplified waves of interest (e.g., the R-waves or P-waves) exceed the threshold level. In response to the amplified EGM signal exceeding the threshold level, the event detector generates an event detection signal. Various methods and apparatus for selecting the sensitivity level and the threshold level are known in the art, including manual physician-adjusted methods and apparatus, and sophisticated microcontroller-based methods and apparatus.
Problematically, however, from the onset of ventricular fibrillation (VF), the amplified cardiac signal will typically begin to exceed the threshold value in a rapid and unpredictable manner due to the rapid, unsynchronized electrical activity in the heart that accompanies VF. Such rapid and unpredictable behavior in the cardiac signal is referred to herein as a VF waveform. In response to the VF waveforms, the event detector will rapidly generate the event detection signal even though no true R-waves are present in the IEGM. That is, the threshold crossings by the VF waveform do not coincide with a synchronous contraction of the heart muscle. Rather than being a wavefront of a global cardiac depolarization (and corresponding contraction), the VF waveform is caused by a relatively small number of cells adjacent to the sensing lead.
Heretofore, various attempts have been made to distinguish between actual R-waves and signals corresponding to VF. The most primitive of such systems, referred to herein as "rate detection systems", simply classify event detection signals that exceed a tachycardia rate threshold (e.g., 200 bpm) as a tachyarrhythmia. U.S. Pat. Nos. 4,181,133; 4,280,502; 4,686,989; 4,969,465; 5,103,822; and 5,205,283; incorporated herein by reference, show such rate detection systems.
Another attempt to distinguish R-waves from VF waveforms (and tachyarrhythmias) is shown in U.S. Pat. No. 4,880,005 ('005 Patent), incorporated herein by reference. In accordance with the '005 Patent, various factors are monitored, such as the rate of detected signals, the stability of such rate, the suddenness in onset of increased rate, and the sustainment of high rate.
Alternately, some methods analyze the morphology (or shape) of the EGM waves to identify or predict VF or tachyarrhythmia. (See, e.g., U.S. Pat. Nos. 4,442,459; 4,552,154; and 4,630,204; incorporated herein by reference.)
Other methods employ complex algorithms in order to try to distinguish between R-waves and VF waveforms or tachyarrhythmias (e.g., U.S. Pat. Nos. 4,493,325; 4,830,006; 4,880,004; 4,905,708; 5,188,105; and 5,217,021; incorporated herein by reference).
For example, the frequency components of the IEGM may be analyzed during a window of time during which the R-wave is expected to occur. In the event that the R-wave is present during the window of time, very high-frequency components will be observed in the IEGM due to the high-frequency content of the R-wave. However, if instead the VF waveform is present in the IEGM during the window of time, relatively low-frequency components will be observed in the IEGM. Using mathematical analysis (e.g., the Fast Fourier Transform, FFT) or other frequency-based algorithm, it is possible to distinguish between an IEGM representing sinus cardiac rhythm, and an IEGM representing ventricular fibrillation.
Another method used to detect VF or VT is the monitoring of several locations in the heart for electrical activity. When R-waves are generated, the electrical activity will occur in the several locations in a specific sequence that is repeated each time the heart beats. At the onset of VF, this sequence will change or become unpredictable, thus indicating that any generated event detection signals are due to VF (see, e.g., U.S. Pat. No. 4,790,317, incorporated herein by reference).
One final example of a method used to distinguish between event detector outputs is shown in U.S. Pat. No. 5,161,527 ('527 patent), incorporated herein by reference. The '527 patent uses at least one metabolic indicator sensor to determine not only the appropriate rate for bradycardia pacing, but also to distinguish physiological from pathological intrinsic cardiac rhythms.
Problematically, each of the methods set forth in the aforecited patents inherently must work backwards to discriminate between event detection signals that correspond to R-waves from those that are a result of VF waveforms or ventricular tachycardia (VT).
What is needed, therefore, is an improved R-wave event detection system which can discriminate between "true" R-waves (i.e., of sinus origin) from ventricular fibrillation.